Background: The GGGGCC (G4C2) repeat expansion in the human open reading frame 72 on chromosome 9, C9orf72, is the most common cause of amyotrophic lat-eral sclerosis (ALS). Studies in transgenic mouse models have linked the pathogenic mechanism of G4C2 repeat expansion to RNA foci or the accumulation of unnatural dipeptide repeats in neurons. However, only one of the existing transgenic mouse lines developed typical ALS. Methods: C9orf72 knockin rats were generated by knockin of 80 G4C2 repeats with human flanking fragments within exon1a and exon1b at the rat C9orf72 locus. Protein expression was detected by west blot. Motor coordination and grip force were measured using a Rotarod test and a grip strength test. Neurodegeneration was as-sessed by Nissl staining with cresyl violet. Results: C9orf72 haploinsufficiency reduced C9orf72 protein expression 40％ in the cerebrum, cerebellum and spinal cords from knockin rats (P?